#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);
    int num[n];
    int i, j;
    int sum1 = (n + 1) * n / 2;
    int sum2 = 0;
    for (i = 0; i < n; i++)
    {
        scanf("%d", &num[i]);
        sum2 += num[i];
    }
    printf("%d", sum1 - sum2);
    return 0;
}